Hydrophilic foam

ABSTRACT

MDI-based prepolymers are blown with a substantially nonaqueous blowing agent, such as pressurized air, and polymerized with stoichiometric amounts of polyoxyethylene polyol having at least two hydroxyl equivalents per mole, yielding a hydrophilic foam. The present foams may be extruded, knife-coated or otherwise cast into sheets, or may be fabricated by other known foam preparation techniques. Because the foam is polymerized with polyoxyethylene polyol instead of water, the foam exhibits both superior drape and improved stretch and recovery as compared with prior-art MDI-based flexible foams formed with aqueous reactants. The foam is particularly suited for use in external biomedical applications as, for example, a laminated medical/surgical dressing in which a thin sheet of the hydrophilic foam adheres to a nonstick aluminized veil on one side.

This application is a division of application Ser. No. 707,955, filedMar. 4, 1985, now U.S. Pat. No. 4,603,076.

FIELD OF THE INVENTION

The present invention relates generally to polyurethane foams andspecifically to an easy-to-handle system for preparing improvedhydrophilic flexible foams which demonstrate superior drape, stretch andrecovery. The foams of the present invention are particularly suited foruse in external biomedical applications.

DESCRIPTION OF THE PRIOR ART

To those familiar with the commercial production of polyurethane foams,the chemistry of the aqueous 2-stage (prepolymer) process is well known.A urethane prepolymer (the reaction product of an isocyanate and apolyol) is reacted with water to generate carbon dioxide. The carbondioxide functions as the blowing agent while simultaneous chainextension and crosslinking cures the prepolymer into a polyurethanefoam. The aqueous 2-stage process has persisted as a foaming techniqueof significant commercial importance for over three decades.

A particular family of polyurethane prepolymers, derived frommethylenediphenyl diisocyanate (MDI) and sold under the trademark HYPOLPLUS, was developed by W. R. Grace & Company for use in the aqueous2-stage process of foam production. These prepolymers, and the aqueous2-step process foams produced therefrom, are disclosed in U.S. Pat. No.4,365,025 to Murch et al., which discloses an isocyanate containingprepolymer in which the isocyanate is a mixture of MDI and polymericforms of MDI. The prepolymer is foamed by mixing it with anapproximately equal amount of water. The resultant flexible foams arecharacterized by greater hydrolytic stability than those foamed fromtolylene diisocyanate (TDI) prepolymers, and the MDI-based foams may bemade without the toxin/carcinogen hazards associated with residual TDIin the workplace.

The product is difficult to handle, however, due to the large volume ofaqueous reactant necessary and due to the high speed of the reaction.Production equipment must be designed to accommodate the introduction ofa substantial quantity of water, must be equipped to evaporate theunreacted portion of the aqueous component (requiring additional energyin the form of heat) and must be capable of producing foamed productslarger than needed to accommodate the severe and uneven shrinking whichoccurs during evaporation. Such production equipment is expensive bothto design and to use. In addition, because a maximum of a few minutes isavailable for fabrication between the time and reactants first commingleand the tack-free cure of the final foam, the product is limited to suchuses as mold casting and foamed-in-place operations and is whollyunsuited to such fabrication techniques as the extrusion andknife-coating processes. For example, none of the MDI-based prepolymersdisclosed in U.S. Pat. No. 4,365,025 to Murch et al. can be mixed withwater and fabricated into a thin foam sheet with a Gardner (or othersuitable) knife: the foam rises and cures long before a thin sheetmaterial can be cast.

Prior to the development of the prepolymers disclosed in Murch et al., amethod was developed for avoiding the handling problems inherent in the2-stage prepolymer process. British Pat. No. 1,306,372 to Marlin et al.teaches a method of mixing an isocyanate with a reactive hydrogencompound in the presence of an organosilicon surfactant, and frothingthe mixture with pressurized inert gas. The organosilicon surfactantprevents the foam from curing by imparting chemical and structuralstability to the froth until the foam is cured by heating. Only latentmetal catalysts may be used in the Marlin et al. process, i.e., thosetin and nickel and other metal catalysts for which organosiliconsurfactants act as synergistic inhibitors at ambient temperatureswithout inhibiting catalytic activity at elevated temperatures. Inaddition to its other chemical properties, the organosilicon surfactantconsistently imparts a hydrophobic character to the final cured product.

Accordingly, the Marlin et al. process is wholly unsuited for use in thepreparation of hydrophilic foams from the Murch et al. prepolymers notonly because the Marlin et al. method requires undesirableelevated-temperature curing and permits only limited uses of surfactantsand catalysts, but because the method cannot yield a hydrophilic foam atall in the presence of organosilicon. A need thus remains for a methodof producing an MDI-based flexible hydrophilic foam which is well-suitedfor use in all types of fabrication techniques and applications, and yetwhich avoids the disadvantages of the aqueous 2-stage prepolymer processand the formulational and end-product limitations of the Marlin et al.technique.

SUMMARY OF THE INVENTION

Without the need for aqueous or organosilicon surfactants and thelimitations they impose, the present invention provides a method forpreparing a hydrophilic foam by blowing an MDI-based prepolymer with asubstantially nonaqueous blowing agent, such as pressurized air, andpolymerizing the prepolymer with a polyoxyethylene polyol having atleast two hydroxyl equivalents per mole. The present method permits thehydrophilic foam to be extruded, knife-coated or cast into sheets, aswell as to be fabricated by other known foam preparation techniques, andthus is suitable for use in any flexible foam operation. Hydrophilicityis controlled with an appropriate non-organosilicon surfactant if asurfactant is used at all, and cure may proceed at ambient or elevatedtemperatures as desired. In addition, because the foam is prepared withpolyoxyethylene polyol instead of water, the foam exhibits both superiordrape and improved stretch and recovery as compared wth prior artMDI-based aqueous 2-stage flexible foams.

DETAILED DESCRIPTION OF THE INVENTION

The prepolymers from which the present foams are prepared are based ondiphenylmethane diisocyanates (for which the common name ismethylenediphenyl diisocyanate, or MDI) and are fully disclosed in U.S.Pat. No. 4,365,025 to Murch et al., incorporated herein by reference.The prepolymers are isocyanate-capped polyols or mixtures of polyolswherein the isocyanate mixture has a functionality of greater than 2.0;the prepolymer is a mixture of diphenylmethane diisocyanate andpolymethylene polyphenyl isocyanate. The prepolymer is mixed with astoichiometric amount of polyoxyethylene polyol, along with certainoptional additives, and a substantially nonaqueous blowing agent foamsthe mixture as it reacts. Because the foamed products of the presentinvention are polymerized with polyoxyethylene polyol instead of water,the foams cure at a lower rate than those prepared by the aqueous2-stage process, yielding a liquid prepolymer/polyol system which curesquicky enough for commercial use, with or without heat curing and withor without a catalyst, yet slowly enough to permit the fabrication offoam products by all known foam fabrication methods, includingknife-coating, extrusion and similar techniques.

The prepolymer and the polyoxyethylene polyol may be mixed together byany mechanical mixing arrangement which will insure complete mixing,such as by mixing in a tank with a high-speed stirrer, by spraying, orby the use of conventional mixing and metering machinery of thepolyurethane industry such as the variable speed mixing head. Thevarious additives are combined with one or the other of the two phases(the prepolymer phase or the polyol phase) prior to mixing. As thephases are mixed, the substantially nonaqueous blowing agent causes theprepolymer/polyol system to rise, and foaming and curing proceed at arate which yields a prepolymer/polyol reaction product in the form of anopen-celled flexible polyurethane foam.

Suitable polyoxyethylene polyols are those having at least about 50% byweight oxyethylene groups. Among the diols are polyethylene glycol typediols having molecular weights from about 200 to about 6,000, such as3,6,9 trioxaundecane 1,11 diol (for which the common name istetraethylene glycol) having a molecular weight of 194. A variety ofsuitable polyoxyethylene polyols may be obtained by the chemicaladdition, known in the art, of ethylene oxide or mixtures of ethyleneoxide and propylene oxide to water or polyhydric compounds. Thepolyoxyethylene triols include, among many, 3,6,9 trioxaundecane 1,7,11triol. The polyoxyethylene polyol may also be a compound having afunctionality of greater than 3, although for reasons of economy andhandling the polyoxyethylene polyol is preferably a polyoxyethylene diolor triol. The amount of polyoxyethylene polyol employed, as comparedwith the prepolymer, will vary slightly depending upon the nature of,and the end use for, the foam being prepared. In general, the totalhydroxyl hydrogen equivalents should be such as to provide a ratio of0.8 to 1.2 equivalents of isocyanate per equivalent of hydroxylhydrogen, and preferably a ratio of 0.9 to 1.1 equivalents of isocyanateper equivalent of hydroxyl hydrogen. The ratio of isocyanate to hydroxylhydrogen, therefore, will always be about 1:1. Foams produced formedical/surgical uses should always be formulated with no more than 1.0equivalent of isocyanate per 1.0 equivalent of hydroxyl hydrogen toeliminate residual unreacted isocyanate (and its associatedcytotoxicity) from the final product.

Pressurized air is a suitable nonaqueous blowing agent for use in thepresent invention, as are other pressurized gases inert to the urethanepolymerization reaction. As the prepolymer and polyol phases are mixed,pressurized air may be injected into the mixture by means of an airpump, or other source of pressurized air, in combination with a suitableair intake valve in the mixing apparatus. If the air is mixed into theprepolymer/polyol system, the air divides into a dispersion of airbubbles which is partially responsible for the cellular structure in thecured foam. The pressurized air need not be entirely nonaqueous but needbe only substantially nonaqueous; although water vapor in the air mightreact with available isocyanate to generate carbon dioxide and result ina minor number of urea linkages in the cured foam, the presence of aminor amount of water vapor will not appreciably increase the rate ofcure of the foam or measurably affect the properties of the finalproduct.

Low-boiling point liquids (containing little or no water) are alsosuitable as substantially nonaqueous blowing agents for the purposes ofthe present invention. These low-boiling point liquids are used in placeof or in conjunction with the injection of pressurized air into theprepolymer/polyol system. Suitable low-boiling point liquids includeesters, ketones, alkanes, chlorinated hydrocarbons and benzenederivatives. Toluene is useful as a blowing agent, as are the loweralkyl acetates such as ethyl acetate. Other suitable low-boiling pointliquids are fluorotrichloromethane, dichlorodifluoromethane andmethylene chloride. These low-boiling point liquids, after incorporationinto the prepolymer/polyol system, expand upon heating to result incommensurate expansion of the polyurethane before and during cure. Thelow-boiling point liquids must be substantially nonaqueous in likemanner as the pressurized air blowing agent, i.e., no more than a minoramount of water may be present in the blowing agent as it is added tothe prepolymer/polyol system.

Because a substantially nonaqueous blowing agent is added to theprepolymer/polyol system, and because the prepolymer/polyol systemitself is substantially nonaqueous, the urea linkages characteristic ofaqueous 2-stage process foams are largely absent from the presentinvention. As a result, the present product is a true urethane systemwherein the linkages are predominantly or exclusively urethane linkages.In theory, although applicant does not intend to be bound by such atheory, this true polyurethane system, substantially lacking inpolyurea, yields the superior drape, stretch and recovery characteristicof the present polyurethane foam due to the relatively higher bondstrength of urethane linkages as compared with urea linkages found inaqueous 2-stage process foams. (By contrast, flexible polyurethane foamscontaining polyurea are more brittle than those of the invention.)Furthermore, because the polyoxyethylene polyol has at least about 50%by weight oxyethylene groups, and the MDI-prepolymer likewise containsat least about 50% by weight oxyethylene groups, the final foam hasconsistent elastomeric strength throughout due to oxyethyleneuniformity, contributing to the superior stretch and recovery.

Although the polyurethane foams of the present invention are hydrophilicper se when formulated without surfactant or with a suitable surfactant,additional hydrophilic compounds may be incorporated into the foam toincrease its capacity to absorb aqueous liquids. These hydrophiliccompounds include derivatives of silica, natural and artificial fibersand hydrophilic polymers. In particular, a useful hydrophilic polymer isthe copolymer of 2-propenoic acid (the common name for which is acrylicacid) and the potassium salt of 2-propenoic acid, a white powdermanufactured by Arakawa Chemical (USA) Inc. and sold under the U.S.Registered Trademark ARASORB. The 2-propenoic acid copolymer is suitablefor use as an absorbent additive in the present polyurethane foambecause it holds over 800 times its weight in water. The ARASORB powdermay be used with or without preliminary ball milling to reduce particlesize, and may be incorporated into the prepolymer/polyol system bypremixing it with the polyol phase. Preferably, however, the ARASORB isball milled to a mesh screen between 100 and 400 before use. Up to about3 parts by weight ARASORB may be mixed with about 1 part by weightpolyol in the preparation of the polyol phase of the system. The ARASORBmay also be added to the prepolymer phase, before the two phases aremixed.

A catalyst may be incorporated into the prepolymer/polyol system bypremixing it with the polyol phase. (The catalyst may also be premixedwith the prepolymer phase, but only immediately before the prepolymerand polyol phases are added together.) The catalyst is added in anyamount which yields an open-celled flexible foam and, accordingly, allbut high concentrations of catalyst may be used if a catalyst is used atall. The most common suitable catalysts are the tertiary amines, such asthe alkyl morpholines, triethylamine and a number of diamines, althoughorganometallic compounds are also suitable catalysts for the presentreactive system. In particular, suitable catalysts include n-methylmorpholine, n-ethyl morpholine, trimethylamine, triethylamine,tetramethyl butane diamine, triethylene diamine, dimethylaminoethanol,benzyldimethylamine, dibutyl tin dilaureate and stannous octoate. Thepreferred catalysts are triethylamine and trimethylamine.

The surfactant may be incorporated into the prepolymer/polyol system bypremixing it with either phase, and preferably with the polyol phase.Any anionic, cationic, nonionic or amphoteric surfactant may be used aslong as it does not yield a hydrophobic product; organosilicon and fattyester surfactants may not be used in the present invention. Thesurfactant may be added in an amount up to about 10 parts by weightbased upon 100 parts by weight of the prepolymer/polyol system. Suitablesurfactants include sorbitan trioleate, polyoxyethylene sorbitol oleate,polyoxyethylene sorbitan monolaureate, polyoxyethylene lauryl ether,polyoxyethylene stearyl ether, fluorochemical surfactants such as ZonylFSN by E. I. du Pont and Fluorad FC 170C by 3M, and block copolymercondensates of ethylene oxide and propylene oxide with propylene glycol,such as the PLURONIC surfactants available from BASF Wyandotte.

Although the present MDI-based polyurethane foams are suitable for usein all flexible polyurethane foam applications, the foams haveparticular utility in biomedical applications. The foam is well suitedfor use in anatomic supports and wound dressing materials both becauseit is hydrophilic and because it can withstand steam autoclavesterilization and attack by solvents and microbes. Furthermore, due tothe unlimited fabrication methods available to prepare the presenthydrophilic foams, the foams may be either cast in molds or knife-coatedor otherwise fashioned directly into sheets for use in wound or surgicaldressings. Foam sheets may also be prepared by foaming a polyurethaneslab or bun and skiving (knife-splitting) it by continuously paring theouter surface of the bun. Sheets having uniform cell size throughout maybe prepared either by skiving or by direct sheet casting, and sheetshaving a gradient cell size may be cast by decreasing the amount ofcatalyst in the system. Three-dimensional anatomic supports and otherthree-dimensional structures may be carved foam buns or foamed directlyinto a mold.

The hydrophilic foam sheets of the invention may be used alone as abandaging material, or may be used in combination with other materialsto form a layered bandage or dressing. The present hydrophilic foam maybe incorporated into any layered dressing in which an absorbent layer isnecessary or desirable, and may be incorporated into transdermal drugdelivery systems. The hydrophilic foam is particularly useful incombination with an aluminized veil, which is a gauze or other poroussubstrate onto which has been vacuum deposited a biologically inertaluminum coating. Aluminized veils are known in the art and areavailable from Lohmann GmbH & Co., KG. When the hydrophilic foam sheetis laminated with the aluminized veil and the aluminized side of thelaminate is placed adjacent a wound, the aluminized veil increases thenonstick properties of the dressing and accordingly broadens its overallmedical utility. The foam may be cast directly onto the aluminized veilor may be laminated with the aluminized veil by means of moisture vaporpermeable or porous adhesives known in the art.

The subject foams may be cured at ambient or elevated temperatures,although elevated temperatures minimize curing time. Specifically, thepresent foams should be cured in an oven or other chamber maintainedbetween about 20° C. and 200° C. for a few minutes to 24 hours, andpreferably between about 40° C. and 180° C. for 2 minutes to 8 hours.Lower and higher cure temperatures than those specified may be used, butexcessively low temperatures may yield an unacceptably slow rate of cureif low quantities of catalyst are used, and excessively hightemperatures may cause discoloration.

The following examples illustrate specific embodiments of and methodsfor making and using the invention.

EXAMPLE I

A polyol phase was mixed in a suitable laboratory vessel containing 6.8g. 3,6,9 trioxaundecane 1,11 diol (tetraethylene glycol), 1.0 g.PLURONIC L-62 surfactant, 10.4 g. ethyl acetate, 3.5 g. toluene and 19.9g. 100 mesh screen ARASORB (2-propenoic acid copolymer) powder. Aprepolymer phase was charged to a separate vessel, and contained 47.1 g.HYPOL PLUS 4000 (a diphenylmethane diisocyanate containing isocyanateproduct with a functionality of greater than 2 comprising a mixture ofdiphenylmethane isocyanate and a polymethylene polyphenyl isocyanate).Four drops of triethylamine (0.4 ml., approximately 0.4 g.) were added,with stirring, to the polyol phase and immediately thereafter both thepolyol and prepolymer phases were charged to a mixing vessel and mixedat high speed for two minutes. The mixed prepolymer/polyol system wasdischarged and was knife-coated onto a release liner by a coating knifeset to a 0.080 mil elevation from the release liner. The release linerwas transferred to an oven and cured for 5 minutes at 110° C. and for 10minutes at 153° C. A cured foam sheet resulted having good appearanceand superior drape, stretch and recovery. The foam was removed from therelease liner and samples of the foam were floated on the surface of avessel containing distilled water. When the liner side of the foamcontacted the distilled water, the foam became saturated with waterafter 10-15 seconds. When the skin of the sample contacted the surfaceof the water, saturation was complete after 3 minutes.

EXAMPLE II

The following were charged to a suitable laboratory vessel to yield apolyol phase: 136 g. 3,6,9 trioxaundecane 1,11 diol (tetraethyleneglycol), 20 g. PLURONIC L-62 surfactant, 208 g. ethyl acetate, 70 g.toluene, and 398 g. ARASORB (2-propenoic acid copolymer) powder groundto 100 mesh. Approximately 8 g. of triethylamine were added, withstirring, to the polyol phase. The prepolymer, 942 g. of HYPOL PLUS4000, and the polyol phase were then transferred to a mixing vessel andthe prepolymer/polyol system was mixed at high speed for 4 minutes. Airwas introduced into the prepolymer/polyol system during mixing.

A rectangular aluminum mold (163/4"×12"×11/2") was lined with releasepaper. The prepolymer/polyol system was discharged evenly into the mold.The foam was permitted to expand at room temperature for 15 minutes,after which the foam was removed to an oven to cure for 3 hours at 108°C. and 1/2 hour at 153° C., sequentially. The foam was skived into acontinuous sheet approximately 0.04 (±0.005) mils thick. Waterabsorption in samples of the foam was complete, from either side of thefoam sheet, after 15 seconds.

EXAMPLE III

A thin foam sheet was prepared by the same method as in Example II,except that approximately 8 g. of triethylene diamine were substitutedfor the 8 g. triethylamine. Tack-free cure of the foam was completedafter 10 minutes in a 115° C. oven, and the resultant foam sheets weremore fine-celled in structure than the foams of Example II.

I claim:
 1. A method of preparing a flexible hydrophilic foam havinggood drape, stretch and recovery, comprising:(a) mixing a prepolymer,derived from a diphenylmethane diisocyanate-containing isocyanateproduct with a functionality of greater than 2.0 and a polyol having atleast about 50% by weight oxyethylene groups, with a polyol having atleast two hydroxyl equivalents per mole and having at least about 50% byweight oxyethylene groups, yielding a prepolymer/polyol system whereinthe ratio of the isocyanate equivalents to the total hydroxylequivalents is about 1:1; (b) blowing said prepolymer/polyol system witha substantially nonaqueous blowing agent; and (c) curing saidprepolymer/polyol system.
 2. The method according to claim 1 whereinsaid step of blowing is carried out by injecting substantiallynonaqueous pressurized inet gas into the prepolymer/polyol system. 3.The method according to claim 1 wherein said step of blowing is carriedout by injecting substantially nonaqueous pressurized inert gas andintroducing an auxiliary blowing agent into the prepolymer/polyolsystem.
 4. The method according to claim 1 wherein said step of blowingis carried out by injecting substantially nonaqueous pressurized inertgas and introducing toluene and ethyl acetate into the prepolymer/polyolsystem.
 5. The method according to claim 4 wherein said mixing furthercomprises adding a surfactant to a prepolymer/polyol system.
 6. Themethod according to claim 5 wherein said mixing further comprises addinga catalyst to said prepolymer/polyol system.
 7. The method according toclaim 6 wherein said mixing further comprises adding a means forabsorbing aqueous liquid to said prepolymer/polyol system.
 8. The methodaccording to claim 7 wherein said mixing further comprises adding acopolymer of 2-propenoic acid and 2-propenoic acid potassium salt tosaid prepolymer/polyol system.
 9. The method according to claim 8wherein said curing further comprises casting the prepolymer/polyolsystem.
 10. The method according to claim 8 wherein said curing furthercomprises casting the prepolymer/polyol system and forming a sheettherewith before curing is complete.
 11. The method according to claim 9which further comprises the step of skiving a sheet from theprepolymer/polyol system after curing is complete.
 12. The methodaccording to claim 8 wherein said curing further comprises casting theprepolymer/polyol system onto an aluminized veil before curing iscomplete.
 13. A flexible hydrophilic foam having good drape, stretch andrecovery comprising the blown reaction product of:(a) a diphenylmethanediisocyanate-containing isocyanate product with a functionality ofgreater than 2 comprising a mixture of diphenylmethane diisocyanate andpolymethylene polyphenyl isocyanate; and (b) a polyol having at leastabout 50% by weight of oxyethylene groups and having at least twohydroxyl equivalents per mole, the ratio of the isocyanate equivalentsto the total hydroxyl equivalents being about 1:1.
 14. The flexible foamof claim 13 wherein said reaction product incorporates a surfactant. 15.The flexible foam of claim 14 wherein said reaction product incorporatesa catalyst.
 16. The flexible foam of claim 15 wherein said reactionproduct incorporates a means for absorbing aqueous liquid.
 17. Theflexible foam of claim 16 wherein said reaction product incorporates acopolymer of 2-propenoic acid and 2-propenoic acid potassium salt. 18.The flexible foam of claim 17 wherein said blown reaction product is asheet.